A substantial proportion of the world's hydrocarbon reserves exists in the form of oil sand or tar sand. Throughout this application the term "bituminous sand" is used to include those materials commonly referred to as oil sand, tar sand and the like. One of the extensive deposits of bituminous sand is found along the banks of the Athabasca River in the Province of Alberta, Canada. In treating the tar sand to recover commercially saleable products, it is first necessary to separate the bitumen from the water and sand.
Typically bituminous sands comprise water-wet grains of sand sheathed in films of bitumen, and contain from about 6% to about 20% bitumen, from about 1% to about 10% water, and from about 70% to about 90% mineral solids. The major portion, by weight, of the mineral solids in bituminous sand is quartz sand having a particle size greater than about 45 microns and less than about 2000 microns. The term "solids" is used herein to describe material of inorganic origin such as sand, clay and the like, as distinguished from materials of organic origin such as coke. The remaining mineral solid material found in bituminous sands has a particle size of less than about 45 microns and is referred to as fines. Fines contain clay and silt including some very small particles of sand. The fines content will vary from about 10% to about 30% by weight of the total solid mineral content of bituminous sand. It is not uncommon for the ingredients of bituminous sand to vary from the mentioned concentrations.
Various methods are known for separating bitumen from bituminous sand. Many of these methods involve, as part of the overall separation process, the use of water to prepare slurries from which the coarse solids and portions of the fines are separated by various means such as settling to recover a bituminous froth which contains some of the fines and quantities of coarse solids.
Although the bituminous froths employed as the feed stock for the process of this invention are not necessarily critically dependent on any particular technique of water extraction of bituminous sand, one well known extraction method for preparing such froths particularly suited for the instant invention is commonly referred to as the Hot Water Method. In broad outline this method involves contacting the oil sand in a tumbler with hot water and steam. The water is supplied at a temperature of about 80.degree. C. and in an amount sufficient to produce a slurry containing about 20% to 25% by weight water. The steam is supplied in an amount sufficient to ensure that the slurry temperature is about 80.degree. C. During slurrying the bitumen films are ruptured and a preliminary separation of the sand grains and bitumen flakes takes place. At the same time, air bubbles are entrained in the slurry. More hot water is added to the slurry after it leaves the tumbler. Typically, this might raise the slurry water content to about 50% by weight. The diluted slurry is then introduced into a separator cell containing a body of hot water. The contents of the cell are commonly maintained at about 80.degree. C. In the cell the bitumen particles, which have been attached to air bubbles, tend to rise to the surface of the water body and form an oily primary froth. This froth is recovered in a launder running around the rim of the cell. The coarse sand particles tend to sink to the bottom of the cell and are drawn off as tailings. A middling stream, comprising water, fine solids (minus about 44 microns) and some bitumen, is continuously withdrawn from the cell at a point intermediate its ends. This middling stream is treated in a sub-aerated flotation cell to recover the contained bitumen in the form of secondary froth. The primary and secondary froths are combined and transferred into a holding tank to remove some of the contained water and solids.
Another well-known technique is known as the Cold Water Method in which the separation is accomplished by mixing the sands with a solvent capable of dissolving the bitumen constituent. The mixture is then introduced into a large volume of water, or water with a surface agent added, or a solution of neutral salt in water.
The Hot Water Method, Cold Water Method and others are extensively described in the literature, and do not form part of the present invention. However, these processes, particularly and preferably the Hot Water Method, do produce the feed stock, bitumen froth containing solids and water, which is treated in accordance with the process of this invention. While the composition of the bituminous froth can vary, it typically comprises about 30% by weight water, about 10% solids and about 60% bitumen. Before the bitumen in the emulsion can be treated to recover saleable products, it is necessary to remove at least most of the water therefrom.
Various proposals have been set forth in the prior art for dehydration of such froths or similar emulsions. For example, one such proposal, as exemplified by Canadian Pat. No. 918,091 proposes dehydration by the bituminous froth with a light diluent naphtha, followed by centrifugation of the product to remove the water and solids. This dehydration system however, involves expensive high-wear equipment and results in substantial losses of bitumen and diluent naphtha with the tailings. As a further example, Canadian Pat. No. 792,734 describes a process wherein water is removed from the bituminous froth by thermal dehydration. In this process the emulsion or froth is heated indirectly in an exchanger with steam to vaporize the water, and the water vapour is subsequently flashed off. It is believed that this process has not been pursued mainly because of the difficulty encountered in heating a non-homogenous mixture such as bituminous froth, and subsequent problems with exchanger fouling caused by clay left behind from the froth.
U.S. Pat. No. 3,468,789 discloses a process wherein an aromatic solvent is added to an equal weight of oil emulsions containing appreciable quantities of solids. The solvent, after some time, causes separation of the froth into three layers, i.e. oil/solvent phase, emulsion or interface, and aqueous phase, some or all of which are treated separately. In this proposed process, emulsified oil which is essentially free of solids is dehydrated by distillation. The aromatic solvent acts as an entrainer and removes the water by azeotrope formation. The aromatic solvents described are expensive and are used in large amounts and the three phase separator poses a difficult design problem, which probably limits the practicality of scale-up to commercial size. The patent (U.S. Pat. No. 3,468,789) also proposes to dissolve the oil emulsion with an equal weight of a solvent capable of forming an azeotrope with water, and, without waiting for the solution to separate into layers as aforesaid, to subject it to azeotropic distillation to remove the water. The oil-wet silt then can be removed from the dehydrated oil/solvent/silt solution either by settling or by means of a centrifuge. The silt is freed of traces of oil by washing with solvent and is then stripped of solvent with steam, and discarded. The solvent is stripped from the post-dehydration oil-solvent solution by distillation and the solvent is replaced in the solution by a low cost distillate diluent for pipelining to a refinery. Like the process of Canadian Pat. No. 792,734 this process may have problems with exchanger and distillation column fouling caused by solid materials. Using equal amounts by weight of fairly expensive solvent is of questionable practicality.
Canadian Pat. No. 792,734 also summarizes various other methods or procedures for treating bituminous emulsions or froths, including gravity settling of solids and wwter after dilution with light solvent, such gravity settling but with elevated temperature and pressure, such gravity settling but with the addition of chemicals to reduce the interfacial tension of the system, and electrostatic treatment after dilution with light solvent. However, as understood by me, the various procedures for breaking bituminous emulsions in recovering the bitumen suffer from various practical shortcomings, such as incomplete separation, high cost, operational problems, etc.
In general, it is an object of this invention to provide a simple but improved process for removing water and part of the coarse solids from bituminous froths, particularly those obtained in the Hot Water Method of extraction treatment of oil sands or tar sands.
Other objects and advantages of this invention will become apparent to those skilled in the art, from the ensuing description of preferred embodiments and examples.